Bone fixation system with curved profile threads

1. A bone fastener for use in orthopaedic surgery for fixing an implant to bone, comprising:

a shaft having a first diameter configured to engage bone, the shaft having a first thread formed thereon;

a head adjacent the shaft, the head having a second diameter larger than the first diameter;

a second thread formed on at least a portion of the head, the second thread having a cross sectional profile along a longitudinal axis of the head including peaks located between first and second flanks, wherein the second thread has a curved shape along the profile with a constant radius of curvature in the form of an arc segment of a circle from the first flank to the peak to the second flank with a center point located within the second diameter of the head;

wherein the first and second flanks of the second thread is flanked by planar valleys.

2. The bone fastener of claim 1 , wherein the first thread is separated from the head by a non-threaded portion of the shaft.

3. The bone fastener of claim 1 , wherein each of the first and second flanks have a radius different than a radius of the curved shape of the second thread located between the first and second flanks.

4. The bone fastener of claim 3 , wherein the angular range of the arc segment is between 30° and 200°.

5. The bone fastener of claim 1 , wherein the head has a constant diameter.

6. The bone fastener of claim 1 , wherein the head has a conical or curved shape.

7. The bone fastener of claim 1 , further comprising a recess in the head to receive a tool to rotate the bone fastener.

8. The bone fastener of claim 1 , wherein the first flank and the second flank have equal radii.

9. The bone fastener of claim 1 , wherein a radius of curvature of the arc segment is between 0.05 mm and 3.0 mm.

10. An implant system for fixation of bone comprising:

an implant having an upper surface, a lower surface, and at least one hole extending through the implant; and

a bone fastener comprising a shaft configured to engage bone, the shaft having a first diameter and a first thread formed thereon, a head adjacent the shaft, the head having a second diameter larger than the first diameter, a second thread formed on at least a portion of the head, the second thread having a cross sectional profile along a longitudinal axis of the head including peaks located between first and second flanks, wherein the second thread has a curved shape along the profile with a constant radius of curvature in the form of an arc segment of a circle from the first flank to the peak to the second flank with a center point located within the second diameter of the head;

wherein the first and second flanks of the second thread is flanked by planar valleys.

11. The implant system of claim 10 , wherein the at least one hole comprises a threaded portion to mate with the second thread of the head of the bone fastener.

12. The implant system of claim 10 , wherein each of the first and second flanks have a radius different than a radius of the curved shape of the second thread located between the first and second flanks.

13. The implant system of claim 10 , wherein the at least one hole has a central axis which is oblique relative to a vertical axis of the implant.

14. The implant system of claim 10 , wherein the at least one hole includes an upper unthreaded portion and a lower threaded portion configured to mate with the second thread of the head of the bone fastener.

15. The implant system according of 14 , wherein the threaded portion of the at least one hole tapers inwardly toward the lower surface of the implant.

16. A bone fastener for use in orthopaedic surgery for fixing an implant to bone, comprising:

a shaft having a first core diameter, the shaft configured to engage bone; and

a head formed on the shaft, the head having a second core diameter larger than the first core diameter of the shaft, the second core diameter being the largest core diameter of the bone fastener, the head having a thread on at least a portion of an outer surface thereon and at least on the second core diameter, the thread being formed to correspond to a thread of the implant,

wherein the entire thread has a profile in longitudinal cross section along a longitudinal axis of the head including peaks located between first and second flanks,

wherein the second thread has a curved shape along the profile with a constant radius of curvature in the form of an arc segment of a circle from the first flank to the peak to the second flank with a center point located within the second core diameter of the head, and

wherein the first and second flanks of the thread is flanked by planar valleys.

17. The bone fastener of claim 16 , wherein the constant radius of curvature extends along substantially the entire profile.

18. The bone fastener of claim 16 , wherein each of the first and second flanks have a radius different than a radius of the curved shape of the second thread located between the first and second flanks.

19. The bone fastener of claim 16 , wherein an angular range of the arc segment is between 30° and 200°.